www.elsevier.nl/locate/ica Inorganica Chimica Acta 300–302 (2000) 384–394 Distribution of hydrophilic, amphiphilic and hydrophobic ions at a liquid/liquid interface: a molecular dynamics investigation Fre ´de ´ric Berny, Rachel Schurhammer, Georges Wipff * Institut de Chimie, Uniersite ´ Louis Pasteur, UMR CNRS 7551, 4, rue B. Pascal, 67 000 Strasbourg, France Received 22 September 1999; accepted 25 November 1999 Abstract We report molecular dynamics studies on the interfacial distribution of ionic species of different size, shape and topology at a water/chloroform interface: hydrophilic K + Cl - ,K + SCN - and K + Pic - ions, amphiphilic ammonium NTMA + cations and farnesylphosphate FPH - anions, tetrahedral hydrophobic AsPh 4 + and BPh 4 - ions, with different counterions. Contrasted distributions are observed. The K + Cl - and K + SCN - ions sit almost exclusively in the water phase, but SCN - is less ‘repelled’ than Cl - by the interface. The Pic - anions are partly adsorbed at the interface and dissolved in the water phase where they display remarkable -stacking interactions. Amphiphilic NTMA + cations or FPH - anions adsorb and dilute at the interface. Less expected is the high surface activity of symmetrical AsPh 4 + and BPh 4 - ions, with marked counterion effects. The two ions fully adsorb at the interface in the AsPh 4 + BPh 4 - salt, while in the Na + BPh 4 - or AsPh 4 + Cl - salts, they display an equilibrium between the organic phase and the interface. Crossed comparisons between the different solutions reveal the important role of counterions on the distribution of a given ionic species. These results are discussed in relation to experimental data. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Liquid interfaces; Ion extraction; Molecular dynamics 1. Introduction The interface between two immiscible electrolyte so- lutions (ITIES) displays peculiar solvation properties, compared to the adjacent liquid phases [1–5]. A precise knowledge of ITIES is of interest in many areas of physical chemistry such as electrochemistry, phase transfer catalysis, drug availability, ion separation by liquid/liquid extraction, liquid chromatography, as well as membranes mimetics. What happens at the surface of water has also important implications in environ- mental processes or atmospheric pollution. However, microscopic pictures of this very thin boarder are difficult to obtain by experiment. As quoted recently, ‘the field is still in its infancy: Little is known about the structure of the interface, and most of our secure knowledge relies on thermodynamics’ [6]. Most of the insights come indeed from electrochemical measure- ments of current, potentials, and surface tension, which give information on the concentration (‘activity’) of the ions at the interface [7] and on energetic aspects of adsorption and transfer across the interface [2,8]. Struc- tural features may be investigated by surface spec- troscopy [9,10], X-ray or neutron reflectivity [11], but precise information is rather limited. Molecular dynamics (MD) or Monte Carlo (MC) [12] simulations techniques, which explicitly account for statistical features of solvation contribute to our under- standing of the solution behaviour of ions and provide microscopic pictures in solution. In the early seventies, Clementi performed MC studies of the hydration of ions [13]. In 1983, Impey et al. investigated by MD the solvation and mobility of ions in water [14]. Most of the studies dealt with aqueous solutions, but other solvents (e.g. acetonitrile [15], chloroform [16], methanol, ammonia and methylamine [17], or carbon- ate solvents [18]) were examined as well. Studies in mixed solvents or at the surface of liquids [19] are less * Corresponding author. Tel.: +33-388-416 071; fax: +33-388- 416 104. E-mail address: wipff@chimie.u-strasbg.fr (G. Wipff) 0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 5 6 1 - 7